Tropical Cyclone Report (TCR); Tropical Storm Oscar

Oscar was a short-lived tropical storm that formed unclimatologically over the eastern Atlantic via Cape Verde origin in early October. Oscar did not threaten land.

a. Synoptic History

Oscar developed from a poorly-defined tropical wave. It is difficult to distinguish the date of this wave's exit from the African coast, but based on a couple of surface observations from the Cape Verde Islands and extrapolation, it is estimated to have entered the eastern Atlantic waters on 27 September. Scatterometer data and surface observations showed that this wave produced a slight cyclonic wind shift in the vicinity of the Cape Verde Islands the next day, but there was little organization to the cloud pattern, and the associated convection was rather poorly-organized. Another tropical wave that moved off the coast of Africa on 28 September may have also played a role in the genesis of Oscar by merging with a preexisting area of cloudiness and showers generated by the other wave, although there was little evidence of this wave in the surface wind field. Regardless, satellite pictures showed that a rather large area of disturbed weather had formed a few hundred miles west of the Cape Verde Islands on 29 September. A broad area of low pressure developed in association with the large disturbance that day. The system was subsequently slow to organize, likely due to the unusually large size of the disturbance as well as moderate southwesterly upper-level shear formulated by a mid- to upper-level trough over the central Atlantic. The wave slowed and turned northward on 3 October. Based on satellite pictures, the wave is presumed to have developed into a tropical depression around 1800 UTC that day, centered about 800 miles west of the Cape Verde Islands. The “best track” chart of the tropical cyclone’s path is given in Fig. 1, with the wind and pressure histories shown in Figs. 2 and 3, respectively (to be added in the final rendition). The best track positions and intensities are listed in Table 1 (to be added in the final rendition). At the time of formation, the low-level center of the depression was exposed well to the west of a rather vigorous cluster of deep convection due strong upper-level westerly winds. Based on an ASCAT pass indicating tropical storm force winds in the eastern portion of the circulation, the cyclone strengthened into a tropical storm around 0600 UTC 4 October.

Oscar steadily strengthened in an environment of strong westerly upper tropospheric vertical wind shear induced by an amplifying trough and associated cold front over the central Atlantic; this was due to the persistent convection to the east of the low-level center, which was likely enhanced by upper divergent flow associated with the eastern portion of the trough. Oscar turned northeast ahead of the cold front the next day and reached its peak intensity of 45 kt around 0200 UTC 5 October. All the while, the circulation was elongating, and Oscar became absorbed into the cold front near 1800 UTC that day.

b. Meteorological Statistics

Observations in Oscar (Figs 2 and 3, to be added in the final rendition) include the satellite-based Dvorak technique from the Tropical Analysis and Forecast Branch (TAFB) and the Satellite Analysis Branch (SAB). In addition, objective ADT estimates from University of Wisconsin CIMSS (UW-CIMSS) were also used to ascertain the intensity of Oscar, especially in its early stages. Various microwave data was useful in tracking Oscar's center, as was certain scatterometer data.

The peak intensity assigned to Oscar at 0200 UTC 5 October is 45 kt, and was based primarily upon an ASCAT pass that morning showing a couple of believable 40 kt wind vectors in the convection to the east. Given the well-documented low bias of that particular instrument, it is reasonable to assume that slightly stronger winds were occurring within the circulation at that time. Oscar began to become distorted by the frontal zone shortly thereafter.

c. Casualty and Damage Statistics

There were no damage reports or fatalities associated with Oscar.

d. Forecast and Warning Critique

While Oscar was expected to develop, the precursor disturbance was not identified until 0000 UTC 2 October, at which time development probabilities were immediately assigned to the "high" category, and the disturbance was given a corresponding 60% chance of developing into a tropical cyclone within 48 hours. Genesis expectations reached 80% approximately 24 hours later.

Since Oscar was a short-lived tropical storm, there were very few forecasts to verify, making their evaluation meaningless.